Design approaches, catalytic combustion

The problems encountered in developing catalysts for fully catalytic combustion have led to the development of various design approaches in which the catalyst temperature stays below the combustor outlet temperature. These approaches are described in the following sections. 

The first innovative systems engineering approach is of Osaka Gas Company in Japan. They developed a multiple monolith catalyst design for the gas-turbine combustion of methane. In this design, different materials are used to fulfil different functions within the catalyst. A palladium catalyst is placed at the entrance of the catalyst system to ignite the catalytic combustion reaction and to raise the temperature. This temperature is then enough... 

Currently, two approaches for the design of catalytic combustors are being tested. The first approach, the multi-monolith catalytic combustor, is based on a very active catalyst at the combustor inlet, followed by less active but more thermostable catalyst segments [4). Complete combustion is to be achieved within the monolithic catalyst in this case. TTie second approach, a hybrid combustor, is based on a partial combustion of the fuel in the catalyst, while the remainder of the fuel is converted in a homogeneous combustion zone downstream of the catalyst [5,6]. The advantage of the multi-monolith is its simplicity whereas the hybrid combustor provides a way to limit the temperature of the catalyst, thereby decreasing the demands placed on the catalyst materials. 

The basic problem in the design of a heterogeneous reactor is to determine the quantity of catalyst and/or reactor size required for a given conversion and flow rate. In order to obtain this, information on the rate equaiion(s) and their parameter(s) must be made available. A rigorous approach to the evaluation of reaction velocity constants has yet to be accomplished for catalytic reactions at this time, industry still relies on the procedures set forth in the previous chapter. For example, in catalytic combustion leac-tioas, the rate equation is extremely complex and cannot be obtained either analytically or numerically. A number of equations may result and some simplification is often warranted. As mentioned earlier, in many cases it is safe to assume that the expression may be satisfactorily expressed by the rate equation of a single step. 

It can be concluded from the previous sections that the p>erfect combustion catalyst has not been found yet, and it is most likely very hard to develop. Hence, reaction engineering must help to circumvent the inherent compromise between activity and stability and the limitations of material science as of today. In this section, the principles of the most promising approaches are outlined. Figure 9 shows schematically the three currently most promising approaches in catalytic combustor design. 

A more rational approach would be to utilise the temperature potential more effectively by incorporating the furnace into a gas turbine cycle, perhaps using a catalytic plate reactor or the equivalent This could be achieved with pressurised combustion, in which the furnace acts as the combustor of a conventional gas turbine cycle. The design of the catalytic plate reactor (see Chapter 5) lends itself well to operation at elevated pressure in view of its millimetre-sized gas channels. Thus power may be extracted from the high-grade heat before it is used at a lower temperature to drive the process. 

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